Mine use to be just grey and black with some white. After i switched the sand to black he now has bright yellow/green outlines on his black stripes. So i guess it depends on if you want those colors then use a dark sand if not then use light colors.
I switched to black substrate and now my del has a pale yellow/greenish base. I have noticed that the black pattern is also more darker which looks sweet!
It also depends on whether or not its Farm Raised or WC. Farm raised ones are bred to be more silvery while WC ones retain their beige/tan and green color